This is a proposal to investigate the roles of lactate as an oxidizable substrate and gluconeogenic precursor during exercise. Our operating hypothesis is termed "THE LACTATE SHUTTLE." We hypothesize that during exercise, lactate is formed at sites of high glycogenolysis and glycolysis and reaches sites of high cellular respiration and gluconeogenesis via the interstitium and plasma. Sites of production include fast glycolytic muscle fibers, fibers undergoing epinephrine stimulation,k and possibly, liver (glucagon stimulation). Removal sites include red skeletal muscle fibers, heart, and liver. Moreover, we hypothesize that lactate is a major gluconeogenic precursor essentially important for the maintenance of blood glucose homeostasis. Moreover we hypothesize that the lactate shuttle provides a mechanism through which metabolism within and between tissues is organized and regulated. In particular, we plan to study how acute and chronic exercise (training) affects the lactate shuttle. For these purposes, we propose a series of kinetic isotope tracer studies, as well as studies using infusion of unlabeled materials and blockers and inhibitors in laboratory rats. By this means we hope to elucidate the hormonal and other signals coordinating the shuttle mechanism. These studies will be complemented by studies using the techniques and approaches of exercise physiology and biochemistry. We plan to isolate sarcolemmal vesicles and to describe tissue differences and training effects on the lactate transporter. We plan also to determine if mitochondrial LDH differs between muscle fiber types and responds to training in ways to explain functioning of the lactate shuttle. Moreover, the activities of glycolytic, TCA cycle and electron transport chain enzyme and components in skeletal muscle, heart and liver will be studied in vitro to provide a mechanistic explanation of metabolite kinetics in vivo.